As a valve device, there is one adapted to include a valve body provided contactably with and separably from a valve seat, and an actuator that displaces the valve body, such as a piezo element, and control a valve opening level by controlling drive voltage to be applied to the actuator.
In recent years, in order to improve the closing performance of a valve device, there has been proposed a valve device configured to include a resin member on the seating surface of a valve body. However, in this configuration, under a high temperature environment, the resin member expands toward a valve seat side, and a reduction in flow rate is likely to increase (see FIG. 4). FIG. 5 illustrates a change in flow rate observed when the surrounding temperature of the valve body was changed from 25° C.→60° C.→25° C. in the configuration adapted to include the resin member on the seating surface of the valve body. Note that FIG. 5 illustrates the results of testing four types of valves. It turns out from FIG. 5 that at 25° C., a variation in flow rate is small, and good reproducibility of a flow rate is obtained. In addition, applied voltage to each piezo element as an actuator was 100 V, and the gauge pressure of fluid supplied to each valve was 50 kPa. As can be seen from FIG. 5, the phenomenon that even when the same drive voltage was applied as that in the 25° C. environment, a flow rate reduced under the 60° C. environment was observed.
Note that in a conventional valve device, the upper limit value of an operable range of drive voltage to be applied to an actuator is fixed in many cases. In order to flow fluid at a full scale flow rate within an operable temperature range (e.g., 25° C. to 60° C.), the upper limit value of the operation range is set to a value obtaining by adding a margin for allowing individual differences to drive voltage for flowing the fluid at the full scale flow rate under a reference temperature (e.g., 25° C.) environment. Note that the full scale flow rate refers to the maximum flow rate at which the fluid can be flowed through a mass flow controller configured to incorporate the valve device, and also refers to a flow rate lower than a flow rate at which the fluid can be flowed when the valve device is in a fully open state.
As for the upper limit value of the operable range, it is conceivable that a value including a margin for allowing thermal expansion of a valve body occurring when the valve device is used in the above-described high temperature (e.g., 60° C.) environment is set as the upper limit value.
However, when the upper limit value of the operable range is the value allowing the thermal expansion under the high temperature (e.g., 60° C.) environment, controlling at the full scale flow rate under a normal temperature (e.g., 25° C.) environment results in an increase in overshoot caused by a transient response, thus deteriorating responsiveness (see FIG. 6). In addition, there is also a problem that the control accuracy (resolution) of flow rate control at normal temperature deteriorates.